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Dislocation structure of fatigue crack tip
Fatigue Abstracts the major influential factors in both the fatigue threshold and fatigue crack growth rate of the alloy. The effect of stress ratio on da/dN is consistent with that on the conventional materials. According to observations of SEM fractographs together with the fatigue crack growth rate measurements, a brief discussion on the fatigue crack propagation behavior was made. Graphs, photomicrographs, 14 ref.
Dislocation structure of fatigue crack tip. Zheng, ES., Wang, Z.G. and Ai, S.H. Actu. Metall. Sin. (China) 11995) 31 (3), A 1 0 5 - A I I 4 (in Chinese) It was observed first in this test that the stabilized dislocation cell and wall structures had been formed on near fatigue threshold of physical short-crack in dual-phase steels. There was a probability of forming dislocation cells on ferfite-martensite interfaces in long-crack growth threshold, where the dislocation lines were denser; there were dislocation lines of single (R=0, x l ) and cross-slip (R=×I) in the second stage of long crack growth; there were obviously rare and lengthened dislocation lines of single and cross-slip in the third stage of long crack growth. It is indicated that the dislocation morphologies of fatigue crack tip were the products of strain history. The dislocation cell and wall structures were substable on the near physical short-crack threshold, it became a dynamic balance with cyclic stress-strain on the threshold, and also was one of the microstructural parameters. Photomicrographs, 14 ref.
Creep fatigue. III. Diercks equation: modification and applicability. Goswami, T. High Temp. Mater. Process (Jan. 1995) 14 (1), 3 5 - 4 5 Creep-fatigue data of low alloy steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Part I while methods of life prediction and their trends were examined in Part II of this work. Diercks equation, a multivariate creep-fatigue life extrapolation equation, for SS 304, in terms of strain range, temperature, hold time and strain rate parameters is modified and extended to the life prediction of low alloy steels in Part 111. Graphs. 14 ref.
Creep fatigue. 11. Creep fatigue life prediction: methods and trends. Goswami. T High Temp. Mater. Process (Jan. 1995) 14 (I), 2 1 - 3 4 Creep-fatigue data of low allow steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Part 1 of this work. Part 11 reviews the methods of creep fatigue life prediction that were assessed from the compiled data. The methods reviewed are phenomenological in nature. However, an empirical method is also discussed which offers very high reliability of the prediction capability. Various test requirements, from which material parameters are determined for each method, have been tabulated. No single method was generalized as a best method of life prediction for all types of creep-fatigue test conditons. Graphs, 43 ref.
Creep fatigue. 1. Compilation of data and trends in the creep-fatigue behavior of low alloy steels. Goswami, 7~ High Temp. Mater. Proce.~s (Jan. 1995) 14 (I), I 20 An attempt to compile the creep-fatigue data of low alloy steels is described. International data have been collected to compare the variability that exists in a particular low alloy steel when characterized in different laboratories. From this work of compilation, trends in the creep-fatigue behavior of low alloy steels were identified, The creep-fatigue behavior, in general, improved with the incase in chromium content; however, when additional alloying elements were added to a standard alloy system, the creep-fatigue behavior of that alloy deteriorated. There was a threshold temperature limit as well as a threshold hold time beyond which only interactions of creep-fatigue and oxidation occurred and reduced the life considerably. However, limiting values of threshold temperature and hold times for different low alloy steels have not yet been determined. Graphs, 24 ref.
345
The double slip plane model for the study of short cracks. Weertman, J., Caracostas, C.A. and Shodja, H.M. Mech. Mater. 11995) 20 (3), t95-208 Apart from crack closure which has been used extensively to explain apparent differences in the behavior of short and long cracks, dislocation crack tip shielding is investigated as an additional contributing factor. The double slip plane (DSP) crack model is used and a numerical approach is followed for the calculation of the dislocation density distributions on the crack and the slip planes and the plastic zone size. By accounting for dislocation crack tip shielding, the critical stress ~r~, and applied stress intensity factor KA~ which define the onset of crack advance, exhibit a crack length dependence which is a manifestation of the anomalous short crack behavior. It is shown, that below a cricial crack length KA~ is not constant, but rather decreases with decreasing crack length. Furthermore, a critical stress smaller than that predicted by linear elastic fracture mechanics is found. A method to calculate crack propagation rates based on dislocation density distributions is also presented. It is shown that under cyclic loading, a short crack propagates below the threshold stress intensity of a long crack and exhibits faster crack growth rates for the same nominal crack driving force. For the mild crack growth regime, Paris power-law exponents of 2 and 2.7 are predicted, depending on the value of an adjustable parameter in the configuration of the DSP model. Graphs, 35 ref.
Optimization of the material properties of titanium turbine blades used in steam turbines (Optimierung der Werkstoffeigenschaften von TitanTurbinenschaufein fur den Einsatz in Dampfturbinen) Koren, M., Puschnik, H. and Fladischer, J. Berg Huttenmann, Monatsh. 11995) 140 (1), 8 8 - 9 5 (in German) Titanium-blades enable an increase in efficiency. The scatter range of structural and material property variations inTi 6AI-4V can be narrowed by allowing a bimodal structure. The average primary a-grain size should be limited to 20 txm (and, by further R&D, to 15 t~m). The percentage of primary c~-grains should be 25-45%, with the grain size being previously adjusted in the bar material. The scatter range of the Young's modulus is B3%. The fatigue strength increases to 450MPa. A further fatigue stength increase can be achieved, under certain conditions, by adjustment of texture, quenching parameters and precipitation hardening parameters.
Fatigue strength of high-strength fine-grained structural steels in the flame cut condition. Kaufmann, I., Schonherr, W. and Sonsino, C.M. Schweissen Schneiden (1995) 3, E46-E51 Because of their high strength characteristics fine-grained structural steels are being used increasingly in general steel construction and in crane and shipbuilding. Because there are so many steels available, in practice the designer is faced with the task of matching to the best advantage the possible cutting speeds and oxy-fuel flame cutting methods in terms of their fatigue strength in the flame-cut condition. Likewise, undue stresses on the flame-cut surface, such as plastic deformation or even damage by impact notches or due to defects during flame cutting, often have to be taken into account when dimensioning components. The fatigue strength after flame cutting, taking into account structural constitution, roughness, hardness and residual stress curves on STE355, StE 460, StE 690 and StE 8911 are studied. Graphs, 38 ref.
Influence of material and mechanical properties on thermal fatigue life of aluminum castings. Tohriyama, S. and Kumano, M. Proc. Aluminum Applications fi~r Automotive Design, Detroit. MI, USA I27 Feb.-2 Mar. 1995) 47 57 There is an ever increasing need for weight reduction and high performance of engine (clean smoke and improving fuel economy). To achieve this, aluminum castings are used for engine parts such as cyliner heads that construct the combustion chamber and are required for thermal resistance. Thermal fatigue tests of AI (319 and 356) castings that are made under various conditions of cooling rate during solidification, heat treatment, and chemical compositions are described. It further investigates the influence of material (such as cooling rate. chemical composition, and heat treatment) and mechanical properties (such as o'B, ~, E/ on thermal fafigue life uf AI castings. Graphs, photomicrographs, 3 ref.
A two-dimensional micromechanic fatigue model. AItus, E. and Herszage, A. Mech. Mater. (May 1995) 20 (3), 209-224 A 2-D cohesive micromechanic fatigue model is proposed. The material is represented by uniaxial elements equally spread in all directions and having a statistical strength distribution. The model is based on two simple microscale assumptions: (a) an interference between a broken element and its vicinity exists, such that the neighbor element may lose some of its strength for the rest of the material's life, and (b) the material microstructure leads to a specific probability function for the neighbor direction. These microscale concepts inherently lead to the basic fatigue behaviors seen from experiments: the S - N power law and the endurance limit. Further properties of the model that are analyzed and discussed are: (a) an inherent directional damage evolution (loading dependent anisotropy), (b) fatigue failure envelopes for biaxial loading, (c) the appearance of cracks parallel to the loading direction under uniaxial compression, and (d) the effect of initial damage on fatigue life. By representing folded macromolecules in polymers as bundles of parallel elements, a relation between the directional probability parameter (and, therefore, the macro fatigue response) and the molecular weight is found and discussed. Graphs, 37 ref
Evaluation of 6013-T6 sheet for damage tolerance applications. 't Hart, W.G.J. and Schra, L. Natl. Lucht Ruimtevaartlab. Rep., Netherlands An experimental program was performed on 1.6 mm thick 6013-T6 and 2024T3 AI clad sheet to compare the residual strength, the fatigue crack growth behavior, and the corrosion properties. The purpose of the investigation was to evaluate whether 6013-T6 can replace 2024-T3 Alclad in damage tolerant structures. The main conclusion was that 6013-T6 cannot replace 2024-T3 Alclad owing to inferior constant amplitude fatigue crack growth resistance. Furthermore, the insufficient resistance to corrosion requires a clad layer that decreases the weight gain over 2024-T3 Alclad. Graphs, photomicrographs, 9 ref.
Evolution of grain boundary serration in fatigue. Tanaka, M. J. Mater. Sci. Lett. 11995) 14 (6), 381-383 The change in the fractal dimension of the grain bmmdary was examined in fatigue specimens (100 mm length, l0 mm width and approx I mm thickness)
Dislocation structure of fatigue crack tip. Zheng, ES., Wang, Z.G. and Ai, S.H. Actu. Metall. Sin. (China) 11995) 31 (3), A 1 0 5 - A I I 4 (in Chinese) It was observed first in this test that the stabilized dislocation cell and wall structures had been formed on near fatigue threshold of physical short-crack in dual-phase steels. There was a probability of forming dislocation cells on ferfite-martensite interfaces in long-crack growth threshold, where the dislocation lines were denser; there were dislocation lines of single (R=0, x l ) and cross-slip (R=×I) in the second stage of long crack growth; there were obviously rare and lengthened dislocation lines of single and cross-slip in the third stage of long crack growth. It is indicated that the dislocation morphologies of fatigue crack tip were the products of strain history. The dislocation cell and wall structures were substable on the near physical short-crack threshold, it became a dynamic balance with cyclic stress-strain on the threshold, and also was one of the microstructural parameters. Photomicrographs, 14 ref.
Creep fatigue. III. Diercks equation: modification and applicability. Goswami, T. High Temp. Mater. Process (Jan. 1995) 14 (1), 3 5 - 4 5 Creep-fatigue data of low alloy steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Part I while methods of life prediction and their trends were examined in Part II of this work. Diercks equation, a multivariate creep-fatigue life extrapolation equation, for SS 304, in terms of strain range, temperature, hold time and strain rate parameters is modified and extended to the life prediction of low alloy steels in Part 111. Graphs. 14 ref.
Creep fatigue. 11. Creep fatigue life prediction: methods and trends. Goswami. T High Temp. Mater. Process (Jan. 1995) 14 (I), 2 1 - 3 4 Creep-fatigue data of low allow steels were compiled from international sources, and trends in creep-fatigue behavior were identified in Part 1 of this work. Part 11 reviews the methods of creep fatigue life prediction that were assessed from the compiled data. The methods reviewed are phenomenological in nature. However, an empirical method is also discussed which offers very high reliability of the prediction capability. Various test requirements, from which material parameters are determined for each method, have been tabulated. No single method was generalized as a best method of life prediction for all types of creep-fatigue test conditons. Graphs, 43 ref.
Creep fatigue. 1. Compilation of data and trends in the creep-fatigue behavior of low alloy steels. Goswami, 7~ High Temp. Mater. Proce.~s (Jan. 1995) 14 (I), I 20 An attempt to compile the creep-fatigue data of low alloy steels is described. International data have been collected to compare the variability that exists in a particular low alloy steel when characterized in different laboratories. From this work of compilation, trends in the creep-fatigue behavior of low alloy steels were identified, The creep-fatigue behavior, in general, improved with the incase in chromium content; however, when additional alloying elements were added to a standard alloy system, the creep-fatigue behavior of that alloy deteriorated. There was a threshold temperature limit as well as a threshold hold time beyond which only interactions of creep-fatigue and oxidation occurred and reduced the life considerably. However, limiting values of threshold temperature and hold times for different low alloy steels have not yet been determined. Graphs, 24 ref.
345
The double slip plane model for the study of short cracks. Weertman, J., Caracostas, C.A. and Shodja, H.M. Mech. Mater. 11995) 20 (3), t95-208 Apart from crack closure which has been used extensively to explain apparent differences in the behavior of short and long cracks, dislocation crack tip shielding is investigated as an additional contributing factor. The double slip plane (DSP) crack model is used and a numerical approach is followed for the calculation of the dislocation density distributions on the crack and the slip planes and the plastic zone size. By accounting for dislocation crack tip shielding, the critical stress ~r~, and applied stress intensity factor KA~ which define the onset of crack advance, exhibit a crack length dependence which is a manifestation of the anomalous short crack behavior. It is shown, that below a cricial crack length KA~ is not constant, but rather decreases with decreasing crack length. Furthermore, a critical stress smaller than that predicted by linear elastic fracture mechanics is found. A method to calculate crack propagation rates based on dislocation density distributions is also presented. It is shown that under cyclic loading, a short crack propagates below the threshold stress intensity of a long crack and exhibits faster crack growth rates for the same nominal crack driving force. For the mild crack growth regime, Paris power-law exponents of 2 and 2.7 are predicted, depending on the value of an adjustable parameter in the configuration of the DSP model. Graphs, 35 ref.
Optimization of the material properties of titanium turbine blades used in steam turbines (Optimierung der Werkstoffeigenschaften von TitanTurbinenschaufein fur den Einsatz in Dampfturbinen) Koren, M., Puschnik, H. and Fladischer, J. Berg Huttenmann, Monatsh. 11995) 140 (1), 8 8 - 9 5 (in German) Titanium-blades enable an increase in efficiency. The scatter range of structural and material property variations inTi 6AI-4V can be narrowed by allowing a bimodal structure. The average primary a-grain size should be limited to 20 txm (and, by further R&D, to 15 t~m). The percentage of primary c~-grains should be 25-45%, with the grain size being previously adjusted in the bar material. The scatter range of the Young's modulus is B3%. The fatigue strength increases to 450MPa. A further fatigue stength increase can be achieved, under certain conditions, by adjustment of texture, quenching parameters and precipitation hardening parameters.
Fatigue strength of high-strength fine-grained structural steels in the flame cut condition. Kaufmann, I., Schonherr, W. and Sonsino, C.M. Schweissen Schneiden (1995) 3, E46-E51 Because of their high strength characteristics fine-grained structural steels are being used increasingly in general steel construction and in crane and shipbuilding. Because there are so many steels available, in practice the designer is faced with the task of matching to the best advantage the possible cutting speeds and oxy-fuel flame cutting methods in terms of their fatigue strength in the flame-cut condition. Likewise, undue stresses on the flame-cut surface, such as plastic deformation or even damage by impact notches or due to defects during flame cutting, often have to be taken into account when dimensioning components. The fatigue strength after flame cutting, taking into account structural constitution, roughness, hardness and residual stress curves on STE355, StE 460, StE 690 and StE 8911 are studied. Graphs, 38 ref.
Influence of material and mechanical properties on thermal fatigue life of aluminum castings. Tohriyama, S. and Kumano, M. Proc. Aluminum Applications fi~r Automotive Design, Detroit. MI, USA I27 Feb.-2 Mar. 1995) 47 57 There is an ever increasing need for weight reduction and high performance of engine (clean smoke and improving fuel economy). To achieve this, aluminum castings are used for engine parts such as cyliner heads that construct the combustion chamber and are required for thermal resistance. Thermal fatigue tests of AI (319 and 356) castings that are made under various conditions of cooling rate during solidification, heat treatment, and chemical compositions are described. It further investigates the influence of material (such as cooling rate. chemical composition, and heat treatment) and mechanical properties (such as o'B, ~, E/ on thermal fafigue life uf AI castings. Graphs, photomicrographs, 3 ref.
A two-dimensional micromechanic fatigue model. AItus, E. and Herszage, A. Mech. Mater. (May 1995) 20 (3), 209-224 A 2-D cohesive micromechanic fatigue model is proposed. The material is represented by uniaxial elements equally spread in all directions and having a statistical strength distribution. The model is based on two simple microscale assumptions: (a) an interference between a broken element and its vicinity exists, such that the neighbor element may lose some of its strength for the rest of the material's life, and (b) the material microstructure leads to a specific probability function for the neighbor direction. These microscale concepts inherently lead to the basic fatigue behaviors seen from experiments: the S - N power law and the endurance limit. Further properties of the model that are analyzed and discussed are: (a) an inherent directional damage evolution (loading dependent anisotropy), (b) fatigue failure envelopes for biaxial loading, (c) the appearance of cracks parallel to the loading direction under uniaxial compression, and (d) the effect of initial damage on fatigue life. By representing folded macromolecules in polymers as bundles of parallel elements, a relation between the directional probability parameter (and, therefore, the macro fatigue response) and the molecular weight is found and discussed. Graphs, 37 ref
Evaluation of 6013-T6 sheet for damage tolerance applications. 't Hart, W.G.J. and Schra, L. Natl. Lucht Ruimtevaartlab. Rep., Netherlands An experimental program was performed on 1.6 mm thick 6013-T6 and 2024T3 AI clad sheet to compare the residual strength, the fatigue crack growth behavior, and the corrosion properties. The purpose of the investigation was to evaluate whether 6013-T6 can replace 2024-T3 Alclad in damage tolerant structures. The main conclusion was that 6013-T6 cannot replace 2024-T3 Alclad owing to inferior constant amplitude fatigue crack growth resistance. Furthermore, the insufficient resistance to corrosion requires a clad layer that decreases the weight gain over 2024-T3 Alclad. Graphs, photomicrographs, 9 ref.
Evolution of grain boundary serration in fatigue. Tanaka, M. J. Mater. Sci. Lett. 11995) 14 (6), 381-383 The change in the fractal dimension of the grain bmmdary was examined in fatigue specimens (100 mm length, l0 mm width and approx I mm thickness)